It has always been my understanding, that the QNH is the barometric pressure, when set on a subscale of an aircraft altimeter, will give the aircraft's height above mean sea level.
It's "popular understanding", but it's not true. 28ft per 1hPa is true only in perfect ISA conditions. If it's colder, atmosphere "compresses" and 1hPa is less then 28ft. When warmer, it's more.
Treating altimeter values as real altitudes is good for navigational purposes, but not for measuring vertical distances accurately.
QNH will give you the real altitude above MSL
only and always when you are on the ground. The higher you are and the more air differs from ISA, the more QNH-set altimeter will lie you.
BUT:
All procedures, separations, MSA, AMA, MRVA and so on - are already corrected for this issue. The only expception are condtions with ground temperature below -15C (in my country, maybe other limiting values are used elsewhere), when obstacle/terrain clearance altitudes should be increased. There's official table for it at DOC-8168 and a lot of unofficial tables in pilots' suitcases.